HVDC (high-voltage direct current) electrical power transmission uses direct current for the transmission of electrical power. This is an alternative to alternating current electrical power transmission which is more common. There are a number of benefits to using HVDC electrical power transmission.
In order to use HVDC electrical power transmission, it is typically necessary to convert alternating current (AC) to direct current (DC) and back again. To date most HVDC transmission systems have been based on line commutated converters (LCCs), for example, such as a six-pulse bridge converter using thyristor valves. LCCs use elements such as thyristors that can be turned on by appropriate trigger signals and remain conducting as long as they are forward biased. In LCCs the converter relies on the connected AC voltage to provide commutation from one valve to another.
Increasingly however, voltage source converters (VSCs) are being proposed for use in HVDC transmission. HVDCs use switching elements such as insulated-gate bipolar transistors (IGBTs) that can be controllably turned on and turned off independently of any connected AC system. VSCs are thus sometimes referred to as self-commutating converters.
VSCs typically comprise multiple converter arms, each of which connects one DC terminal to one AC terminal. For a typical three phase AC input/output there are six converter arms, with the two arms connecting a given AC terminal to the high and low DC terminals respectively forming a phase limb. Each converter arm comprises an apparatus which is commonly termed a valve and which typically comprises a plurality of elements which may be switched in a desired sequence.
In one form of known VSC, often referred to as a six pulse bridge, each valve comprises a set of series connected switching elements, typically insulated gate bipolar transistors (IGBTs) connected with respective antiparallel diodes. The IGBTs of the valve are switched together to electrically connect or disconnect the relevant AC and DC terminals, with the valves of a given phase limb typically being switched in anti-phase. By using a pulse width modulated (PWM) type switching scheme for each arm, conversion between AC and DC voltage can be achieved.
In another known type of VSC, referred to a modular multilevel converter (MMC), each valve comprises a chain-link circuit having a plurality of cells connected in series, each cell comprising an energy storage element such as a capacitor and a switch arrangement that can be controlled so as to either connect the energy storage element between the terminals of the cell or bypass the energy storage element. The cells are sometimes referred to as sub-modules, with a plurality of cells forming a module. The sub-modules of a valve are controlled to connect or bypass their respective energy storage elements at different times so as to vary over the time the voltage difference across the plurality of cells. By using a relatively large number of sub-modules and timing the switching appropriately the valve can synthesise a stepped waveform that approximates to a desired waveform, such as a sine wave, to convert from DC to AC or vice versa with low levels of harmonic distortion. As the various sub-modules are switched individually and the changes in voltage from switching an individual sub-module are relatively small, a number of the problems associated with the six pulse bridge converter are avoided.
In the MMC design each valve is operated continually through the AC cycle with the two valves of a phase limb being switched in synchronism to provide the desired voltage waveform.
Recently a variant converter has been proposed wherein a chain-link of a series of connected cells is provided in a converter arm for voltage wave-shaping, e.g. providing a stepped voltage waveform as described, but each converter arm is turned off for at least part of the AC cycle. Thus the plurality of series connected cells for voltage wave-shaping are connected in series with an arm switch, referred to as a director switch, which can be turned off when the relevant converter arm is in the off state and not conducting. Such a converter has been referred to as an Alternate-Arm-Converter (AAC). An example of such a converter is described in WO2010/149200.
FIG. 1 illustrates a known Alternate-Arm-Converter (AAC) 100. The example converter 100 has three phase limbs 101a-c, each phase limb having a high side converter arm connecting the relevant AC terminal 102a-c to the high side DC terminal DC+ and a low side converter arm connecting the relevant AC terminal 102a-c to the low side DC terminal DC−. Each converter arm comprises a circuit arrangement 103 of series connected cells, the arrangement 103 being in series with an arm switch 104 and inductances 105. It will be noted that FIG. 1 illustrates a single arm inductance but one skilled in the art will appreciate that the arm inductance may in practice be distributed along the arm between the AC and DC terminals.
The circuit arrangement 103 comprises a plurality of cells 106 connected in series. Each cell 106 has an energy storage element that can be selectively connected in series between the terminals of the cell or bypassed. In the example shown in FIG. 1 each cell 106 has terminals 107a, 107b for high-side and low-side connections respectively and comprises a capacitor 108 as an energy storage element. The capacitor 108 is connected with cell switching elements 109, e.g. IGBTs with antiparallel diodes, to allow the terminals 107a and 107b of the cell to be connected via a path that bypasses capacitor 108 or via a path that includes capacitor 108 connected in series. In the example illustrated in FIG. 1 each cell comprises four cell switching elements 109 in a full H-bridge arrangement such that the capacitor can be connected in use to provide either a positive or a negative voltage difference between the terminals 107a and 107b. The circuit arrangement 103 of such series connected cells can thus operate to provide a voltage level that can be varied over time to provide stepped voltage waveform for wave-shaping as discussed above. The circuit arrangement 103 is sometimes referred to as a chain-link circuit or chain-link converter or simply as a chain-link. In this disclosure the circuit arrangement 103 of such series connected cell for providing a controlled voltage shall be referred to as a chain-link.
In the AAC converter the chain-link 103 in each converter arm is connected in series with an arm switch 104, which will be referred to herein as a director switch, which may comprise a plurality of series connected arm switching elements 110. The director switch of a converter arm may for example comprise high voltage elements with turn-off capability such as IGBTs or the like connected with antiparallel diodes.
When a particular converter arm is conducting, the chain-link 103 is switched in sequence to provide a desired waveform in a similar fashion as described above with respect to the MMC type converter. However, in the AAC converter, each of the converter arms of a phase limb is switched off for part of the AC cycle and during such a period the director switch 104 is turned off.
When the converter arm is thus in an off state and not conducting the voltage across the arm is shared between the director switch and the chain-link circuit. Compared to the MMC type VSC the required voltage range for the chain-link of each converter arm of an AAC type converter is thus reduced, with consequent savings in the cost and size of the converter.
In an HVDC VSC installation one problem that may occur is a flash-over across insulators forming part of a converter system, e.g. across insulators inside the valve-hall. Such events cause a short circuit between parts of the circuit which can result in high-level fault currents that, if not dealt with properly, are likely to destroy the semiconductor switching elements and/or their auxiliary circuitry. Such events can result in significant damage requiring significant repairs with a long and costly down-time.